1. Field of the Invention
This invention relates to polychromatic cathode ray tubes, and in particular to cathode ray tubes which produce an image of varying color in response to variations in the energy imparted to a luminescent screen by a scanning electron beam.
2. Description of the Prior Art
One of the most commonly used polychromatic cathode ray tubes is the shadow mask color display tube. This tube produces images of generally adequate resolution and color accuracy, but both the image brightness and the electrical efficiency of the tube can be improved if the shadow mask is omitted. The shadow mask is provided with apertures which correspond to respective areas of the screen formed of red, green and blue luminescing phosphor materials, and these apertures permit only those portions of scanning electron beams designated red, green and blue which register with the respective color phosphor areas to pass through the mask. The mask not only limits the usable regions of the screen to those corresponding with the apertures, thereby limiting the brightness of the image, but also intercepts a large part of the electron beam current, thereby reducing the electrical power efficiency of the tube.
It is possible to eliminate the mask, to eliminate all but one electron beam, and to utilize virtually the entire screen area for image production by forming the screen from a phosphor material having a composition which differs with depth and which luminesces in different colors depending on the depth which the electrons in the beam penetrate into the screen. U.S. Pat. Nos. 3,371,153, 3,517,243 and 3,560,398 disclose color picture tubes having such penetration type screens which rely on control of the electron velocity to determine the depth of penetration. This velocity is controlled by varying the electron beam accelerating potential as the beam is scanned across the screen, effecting corresponding variations in the color of luminescence. The accelerating potential must be changed by several kilovolts to effect a color change, however, and the rate at which such large changes in voltages can be made is limited by the substantial capacitance of the picture tube's accelerating anode, which covers a large portion of the inner surface of the tube envelope. Although this problem might be avoided by maintaining the anode potential constant and varying the cathode potential over several kilovolts, as is disclosed in U.S. Pat. No. 3,863,097, it would then be difficult to prevent arcing between the cathode and other nearby conductors. Regardless of which electrode is utilized to control the accelerating potential, there remain the difficulties of accurately controlling the velocities of the electrons impinging on the screen and of manufacturing a screen having a composition which varies precisely with depth. The latter difficulty can be avoided by manufacturing the screen from a random mixture of different types of phosphor particles which luminesce in different colors and which are excited at different electron velocities, as is proposed in U.S. Pat. Nos. 3,339,016 and 3,522,368, but this still leaves the problems of rapidly changing the accelerating potential and of accurately controlling the velocities of impinging electrons.
In another type of maskless color display tube, disclosed in U.S. Pat. No. 2,431,088, a screen is formed from a mixture of phosphor materials, each of which luminesces in a different color and produces maximum light emission at a different beam current density. Although this type of tube avoids the difficulty of varying the high voltage accelerating potential and has a simple screen configuration, the use of current density to vary color causes other complications. If the beam current density is varied by varying the area of the beam spot on the screen, image resolution also varies. Resolution improves with decreasing spot size and worsens with increasing spot size. Thus a polychromatic image presented on the display tube screen has non-uniform resolution. Conversely, if the current density is varied by varying the beam current alternative means must be provided for controlling image brightness, which is itself typically controlled by varying the beam current.
The phosphors used to make the screens of prior art picture tubes generally include a powdered base compound to which a small quantity of an activator metal has been added for increasing luminous efficiency. Although some powdered phosphors luminesce efficiently without the use of activators, their use is generally limited to monochromatic displays such as oscilloscopes where a rapid decay of luminescence is desirable. Two phosphors commonly used for this purpose are zinc oxide and calcium tungstate.
Some crystalline semiconductor materials luminesce in visible light when continuously excited with low-energy electrons. Typical examples are GaAs.sub.x P.sub.1-x light emitting diodes which luminesce in red, yellow or green, depending on the amounts of As and P, when current is passed through the diodes. It has also been reported by Zhong and Bryant, J. Phys. C: Solid State Phys., 15 (1982), pages 3411-3423 that zinc selenide (ZnSe) single crystals can be made into electroluminescent diodes which emit blue light varying in wavelength from approximately 4400-4700 .ANG. as the temperature of the crystals is varied from approximately -269.degree. C. to +20.degree. C.